Process for prorducing thallium-containing high-Tc superconductors in flowing gas atmospheres

ABSTRACT

The process for producing high-T c  superconductors containing thallium, calcium, barium and copper, possibly also lead and/or strontium, provides for a thallium-free precursor to be produced in a first reaction step and this to be then mechanically triturated, subsequently heated to temperatures in the range from 700° to 950° C. and heat treated for a period of at least 3 hours. The mixture is then cooled to ambient temperature and ground again. Finally, it is heat treated at temperatures of from 400° to 500° C. in a stream of pure oxygen. The thallium-free precursor is then triturated with Tl 2  O 3 , if desired shaped into a shaped part and then oxidatively fired in a flowing gas atmosphere.

The invention relates to a process for producing thallium-containingoxide-ceramic superconductors, by means of which massive ceramiccomponents and also precursor materials for the "oxide powder in tube"method (OPIT) can be obtained.

It is already known that a plurality of high-T_(c) super-conductingoxide compounds are formed in the pseudoquaternary system Tl₂ O₃/CaO/BaO/CuO. This is primarily dependent on the ratio of the metaloxides used in each case, on the temperatures used and on the oxygenpartial pressure during firing. Unlike the case of the structurallyrelated Bi compounds, in the case of thallium-containing superconductorsthere are two classes of compounds having the general, nominalcompositions:

a) Tl/PbCa_(n-1) Ba₂ Cu_(n) O_(x) (low-thallium) and

b) (Tl/Pb)₂ Ca_(n-1) Ba₂ Cu_(n) O_(y) (thallium-rich).

in the wide sense, a) includes the corresponding barium-free strontiumcompounds Tl₀.1 Pb₀.5 Ca_(n-1) Sr₂ Cu_(n) O_(z).

In the case a) the compounds having n=2 and n=3 and in the case b) thosehaving n=1, 2 and 3 form superconductors having critical transitiontemperatures (T_(c)) above the boiling point of liquid nitrogen (T=77K).The discovery and characterization of these compounds is described, forexample, by Z. Z. Sheng et al. in Nature, vol. 332, p. 55, (1988). Thehighest transition temperature T_(c) known hitherto is 125K for thethallium-rich, lead-free compound of the type Tl-2223 from theabove-mentioned series b). The transition temperatures for Tl-1223 fromthe series a) are in the range 118K<T_(c) <122K. According to thestudies of D. H. KIM et al., published in Physica C 177, p. 431, (1991),there is a clear relationship between the critical current density andthe spacing of the so called CuO planes in the crystal structure, as afunction of the details of the production method.

This is related to the melting behavior of the magnetic flux lattice,particularly in the superconductors built up of layers. In the case ofthe thallium-containing superconductors, the best magnetic couplingbetween the layers is observed in the case of the compounds of theabovementioned low-thallium series a) having n=2 and n=3 see T. DOI etal., Physica C 183, p. 67, (1991)!. At a temperature of 77K and amagnetic field of 1 T, intragranular, critical currents of the order ofJ_(c) =10⁵ A/cm² were able to measured cf. R. S. LIU et al. in Appl.Phys. Lett. 60, p. 1019, (1992)!. The critical transport currentdensities are at present, owing to the grain boundary effects (weaklinks), still far below this value, namely in the region of a few 100A/cm². For the above reasons, the Tl superconductors of the series a)(Tl-1223 and Tl-1122) are of greater practical importance for use inpower applications in the temperature range above 77K. They are evensuperior to the bismuth-containing superconductor ceramics in thistemperature range, because there only well characterized compoundsanalogous to series b), containing Bi in place of Tl and Sr in place ofBa, are known. These are compounds of the nominal composition Bi₂ CaSr₂Cu₂ O_(Y) (n=2, T_(c) =92K) and Bi₂ Ca₂ Sr₂ Cu₃ O_(x) (n=3, T_(c) =110K)in which the spacing between the CuO planes is in principle greater thanin the comparable thallium compounds of the series a).

A further, industrially significant compound is YBa₂ Cu₃ O₇ (T_(c)=92K). Although the spacing between the CuO planes is there even lessthan in the case of the thallium-containing superconductors, the grainboundary effects are significantly more pronounced for a comparabletexture, which is apparent in a stronger magnetic field dependence ofJ_(c).

The actual preparation of oxide ceramic superconductors is carried outby means of the solid state reaction at high temperatures. In the caseof the thallium-containing superconductor ceramics, the propertiesachieved at low temperatures depend essentially on the ratio of themetal atoms weighed out in the starting mixture, the selection of thestarting components themselves, the oxygen partial pressure of thesurrounding gas atmosphere and finally on the temperatures used and theTl₂ O vapor pressure during firing. The preparation of the TlCa₂ Ba₂ Cu₃O₈.5+x super-conducting ceramic is described by K. C. Goretta et al. inSupercond. Sci. Technol. 5, p. 534, (1992). They are accordinglyproduced at high temperatures from pre-calcined mixtures containing thestarting oxides in the desired metal ratio. This occurs finally inhermetically sealed containers (quartz ampoules) or using gold foils toavoid thallium losses at temperatures around 900° C., in anoxygen-containing atmosphere, with a partial melt being formed. Thereaction time is 0.1 h with subsequent slow (3K/min) lowering of thetemperature to 805° C. The preparation of phase-pure formulations isobviously made difficult. Additions of strontium and also of lead canimprove the results cf. S. P. Matsuda et al., Appl. Phys. Lett. 59, p.3186, (1991)!. A disadvantage is that the contamination of the sampleswith BaCuO₂ can hardly be completely suppressed. Contamination withcarbonate likewise reduces the proportion of the superconducting phase.Up to now, the absolutely necessary use of quartz ampoules and goldfoils in particular makes the production of relatively large ceramiccomponents or amounts of powder uneconomical or even impossible inindustry.

It is therefore an object of the invention to provide a process forproducing ceramic parts and powder of thallium-containing high-T_(c)superconductors (Tl-1223 and Tl-1122), in which minimal Tl losses cannevertheless be realised without use of quartz ampoules or containersclosed with gold foil, so as to make the principle advantage of highertransition temperatures and better current capacity (at T>77K) of thethallium-containing superconductors industrially utilizable.

The object is achieved according to the present invention by a processof the generic type for producing high-T_(c) superconductors containingthallium, calcium, barium and copper, possibly also lead and/orstrontium, in which process a thallium-free precursor is produced in afirst reaction step from a stoichiometric mixture of compounds of themetals Ba, Ca and Cu, possibly also Pb and/or Sr, having the metalcontent in the numerical ratio of atoms desired in the particular case,and in which the thallium-free precursor is then triturated with Tl₂ O₃and, if desired, is shaped to give a shaped part, wherein the precursortriturated with Tl₂ O₃ is oxidatively fired in a flowing gas atmosphere.

The superconductor ceramic is formed during the firing in a solid statereaction from an oxide mixture essentially containing thallium, possiblyalso lead, calcium, barium and copper and having the approximate nominalcomposition Tl/PbCa₂ Ba₂ Cu₃ O₈.5+x (abbreviated as Tl-1223) orTl/PbCaBa₂ Cu₂ O₆.5+y (abbreviated as Tl-1122).

In the process of the present invention, a thallium-free precursor, aprecalcined intimate mixture of alkaline earth metal oxocuprates, isproduced in a first reaction step. For this purpose, the mixture of themetal compounds is preferably mechanically triturated, subsequentlyheated to temperatures in the range from 700° to 950° C. and heattreated for a period of at least 3 hours, preferably from 8 to 12 hours.The mixture is then cooled to ambient temperature, again ground andsubsequently heat treated in a stream of pure oxygen at temperatures offrom 400° to 500° C. The risk of contamination of the reaction productby carbonate residues can, according to the present invention, besubstantially reduced by the use of alkaline earth metal hydroxides inplace of the customary carbonates. Commercially available products suchas Ba(OH)₂ ×8 H₂ O and Sr(OH)₂ ×8 H₂ O can, if necessary, be purifiedsufficiently by recrystallization for only carbonate residues from theCa(OH)₂ weighed out to be able to remain.

When using the hydroxides, the major part of the water ofcrystallization is removed on heating the mixture of the alkaline earthmetal hydroxides and the copper oxide, and also possibly the lead oxideor lead nitrate, having the metal content in the numerical ratio ofatoms desired in each case, preferably in a drying oven, at temperaturesof at least 130° C., in particular at 150° C., over a period of at least1 hour, preferably at least 2 hours. This can result in partial meltingin the water of crystallization. Since this melt is very aggressive,chemically inert vessels, advantageously of polytetrafluoroethylene, areused here as evaporating dishes. To completely drive off the water ofcrystallization, this is followed by a further drying process in acorundum crucible in an electric muffle furnace at a temperature of 400°C., preferably 420° C., for a period of from 1 to 3 hours. Under theseconditions, the degradation ends approximately at the stage of themonohydrates Ba(OH)₂ ×H₂ O or Sr(OH)₂ ×H₂ O, which can be checked bymeans of thermoanalytical measurements. The mixture, after beingtriturated in an agate mortar, is then heated to temperatures in therange from 700° to 950° C. and heat treated over a period of at least 3hours, preferably from 8 to 12 hours, with a sintered oxide mixture ofalkaline earth metal oxocuprates being formed. The remaining water isgiven off only above 400° C. or 600° C.

The finished precursor is obtained after grinding and subsequent heattreatment at a temperature T of 400° C.≦T≦500° C. in a stream of pureoxygen.

In a second step, the precursor thus prepared is triturated with thedesired amount of Tl₂ O₃ and, if desired, pressed to form the respectiveshaped bodies.

The subsequent firing process is carried out, according to the presentinvention, in a mobile tube furnace (electric resistance furnace) in aflowing, oxidizing gas atmosphere at a flow rate of ≧3 l/hr and at aheating rate of at least 30K/min, preferably 60K/min. The gas used ispreferably air or pure oxygen. To enable better checking of thetemperature, the tube furnace can be specially equipped with anadditional thermocouple.

FIG. 1 shows a longitudinal cross section through a furnace 13 suitablefor the process of the present invention. Reference numerals indicate anouter quartz tube 1 and an inner quartz tube 2 which are both arrangedin a tubular tunnel 3 which is surrounded on all sides by the electricheat elements 4. In addition, the furnace 13 possesses a first outerthermocouple 5 and a second inner thermocouple 6. In the interior of theinner quartz tube 2 there is arranged a boat 7 containing the sample 8.During the first heating phase, the first outer thermo-couple 5, whichis part of the standard equipment, measures the temperature in thevicinity of the heating winding 4 and at the same time serves for theregulation of the furnace temperature. The second thermocouple 6 isconducted through a central capillary 9 in a ground glass cap 10 andthrough a protective tube 11 of laboratory glass. The protective tube 11is connected to the glass capillary 9 by a connection piece 12 whichpreferably consists of polytetrafluoroethylene. The furnace 13 is fittedwith a thermal insulation 14 which is provided with an outer jacket 15.The outer thermocouple 5 is surrounded by a protective tube 16.

The second thermocouple thus makes it possible to measure thetemperature in the interior of the sample chamber in the direct vicinityof the sample. By means of a selection switch, temperature regulationcan also be carried out when desired by means of this measuring point.

The actual solid state reaction takes place at the firing temperatures.According to the present invention, the sample chamber consists of alonger, outer tube, preferably of quartz glass, having ground glasscones and an inner protective tube. The dimensions depend in each caseon the size and amount of the superconductor ceramic to be produced. Theresistance tube furnace itself is dimensioned such that it can juststill be moved back and forth over the tube.

The firing temperature, i.e. the temperature at which the solid statereaction leading to the superconducting compound takes place, is in eachcase different for the different compositions. Thus, the abovementionedcompounds having the nominal composition Tl/PbCa₂ Ba₂ Cu₃ O₈.5+x(abbreviated as Tl-1223) can be produced in a multistep firing processat temperatures of at least 900° C., preferably of at least 905° C.; inthis process the firing temperature should be maintained for a time ofat most 30 minutes, preferably at most 20 minutes. Other compoundshaving the nominal composition Tl/PbCaBa₂ Cu₂ O₆.5+y (abbreviated asTl-1122) are formed at firing temperatures of only from 760° to 790° C.if this temperature is held for a time of at least 3 hours. The actualfiring process is advantageously followed by an additional furtherthermal treatment at or above 700° C. for a time of at least 5 hours,preferably from 8 to 16 hours.

For the firing process of the present invention, the sample is firstheated at a rate of at least 6K/min to a temperature of 500° C. and isheat treated for a period of from 8 to 12 hours in a flowing gasatmosphere at a flow rate in the range from 3 to 30 l/h. In the case ofthe Tl-1223 compound, the furnace is then again pushed away from thepart of the tube containing the sample and is preheated to a temperatureof at least 1000° C. The first outer thermocouple is adjacent to theheating winding and provides the thermal voltage for the temperatureregulator. The gas stream which flows through the sample chamber,preferably pure oxygen, is again checked and, if necessary, adjusted sothat a good gas flow exists around the sample. The latter is dependenton the dimensions of the sample chamber, preference being given to aflow rate in the range from 10 to 30 l/h.

After reaching the final temperature, the furnace is moved over the partof the tube containing the boat and the green body to be fired. Thefurnace temperature then begins to fall. At a temperature of about 900°C., temperature measurement is switched over to the second innerthermocouple. Temperature monitoring and furnace regulation are fromthen on carried out via the second inner thermocouple. This indicatesthat the sample temperature is at first still below 800° C. After ashort time, the preferred heating rate being 6K/min, the maximumtemperature of 900° C.≦T_(max) ≦915° C., preferably 905° C., is reached.This results in formation of the abovementioned partial melt. It can beestablished by means of thermoanalysis that the weight loss of thesample per unit time is most pronounced in this process stage. Afterreaching the reaction temperature, the temperature is lowered again tofrom 710° to 720° C. at a rate of from 4 to 6K/min, preferably 6K/min.In this temperature range, the further treatment is carried out for aperiod of from 8 to 16 hours, preferably 12 hours, preferably in a pureoxygen atmosphere. The superconducting ceramic is obtained afterremoving the tube furnace from the part containing the sample during thecooling phase, which is as rapid as possible, to ambient temperature.The samples can still be significantly contaminated with unreactedoxocuprates.

Naturally, the proportion of BaCuO₂ shows up particularly in X-raypowder studies. The weight losses during firing are in the range between3 and 15% by weight, typically 3% by weight. A further improvement inrespect of the maximum achievable density of the superconductor can,according to the present invention, be achieved by means of twoadditional measures.

a.) The actual firing process including further treatment and cooling isrepeated a number of times, preferably two or three times, but withoutthe preignition at 500° C.

b.) The ceramic is reground once more, mixed with from 1 to 3% by weightof Tl₂ O₃, possibly also only now with the lead oxide or PbNO₃, againpressed and, as mentioned above, fired. This may be followed by theprocedure a.).

For the preparation of the superconductor of the type Tl-1122, it isalso possible according to the present invention to use another variantof the firing process in which the maximum temperature is only 780° C.In this variant, the furnace does not have to be preheated. The lowerfiring temperature offers the advantage of weight losses which are onlysmall. The firing process can take a number of hours at thistemperature, without resulting in a practically significant change inthe nominal composition weighed out due to weight decrease. The measuresa.) and b.) can be omitted in this variant.

According to the present invention, a copper-rich starting mixture basedon the idealized numerical ratio of atoms of the formulae TlCa₂ Ba₂ Cu₃O₈.5+x and TlCaBa₂ Cu₂ O₆.5+y respectively is used. The copper excess isin the range from 0.2 to 1.5 relative atomic mass units. An excess ofcalcium and a deficiency of barium are weighed out. The ratio of thenumbers of atoms of Ca/Ba, or possibly Ca/Σ(Ba+Sr), is greater than 1.1to above 1.23, preferably 1.2. The strontium addition is then from 0.1to 0.2 relative atomic masses, but not in the case of the compounds ofthe type Tl₀.5 Pb₀.5 Ca_(1-n) (Ba/Sr)₂ Cu_(n) O_(z) if a large part ofthe Ba is here replaced by Sr. A typical example is given by the formulaTlCa₂.09 Ba₁.73 Sr₀.18 Cu₃.44 O_(x). Standardized to 3 copper atoms,this is then Tl₀.87 Ca₁.82 Ba₁.51 Sr₀.16 Cr₃ O₈.5+x. If lead is to beadded to stabilize the Tl-1223 phase, the proportion is from 0.05 to 3%by weight, calculated as lead oxide. When weighing out the startingmaterials for producing the compound of the type Tl-1122, with thereduced calcium or copper content, the corresponding calcium or copperexcess is analogously taken into account (e.g. Tl₀.8 Ca₁.2 Ba₂ Cu₂ O_(z)instead of TlCaBa₂ Cu₂ O_(z)).

The process of the present invention is particularly suitable forproducing sintered bodies of a wide variety of dimensions, depending onthe dimensions selected for the tube furnace used. The criticaltransition temperatures are in the range 105K≦T_(c) ≦116K for Tl-1223,and in the range 93K≦T_(c) ≦105K for Tl-1122. The sintered bodies canhave contacts applied to them in a downstream process, by smoothing thesurface (grinding or polishing) and subsequent etching and sputtering onof silver. After firing on, for example, a silver layer at T=500° C. (inO₂), contacts suitable for power applications and having contactresistances of R=10 μΩ are obtained. Crystal powders as precursors forthe OPIT method can be produced by grinding the reacted compacts.

In the following examples, the invention is further clarified to thoseskilled in the art without being restricted to the concrete embodimentsdescribed.

EXAMPLE 1

63.07 parts by weight of Ba(OH)₂ ×8 H₂ O, 16.28 parts by weight ofCa(OH)₂ and 23.87 parts by weight of CuO were weighed out, mixed andfirst heated in a drying oven for a time of 3 hours at a temperature of150° C. The mixture was then triturated in an agate mortar and heated ina muffle furnace in a corundum crucible first for a time of 3 hours at atemperature of 420° C. and then held for a further 3 hours at from 600°to 650° C. After cooling, the mixture was again triturated well. Themetal oxide mixture obtained was then heated in a muffle furnace in airto a temperature of from 900° to 950° C. and heat treated for a periodof 10 hours. The sintered mixture of various oxocuprates was ground in amortar and heat treated in a pure oxygen atmosphere for 1 hour attemperatures in the range from 400° to 450° C. 18.27 parts by weight ofTl₂ O₃ were then weighed out and added, the oxide mixture was mixed inan agate ball mill with a little petroleum spirit (boiling range from60° to 90° C.) to form a thin slurry and was ground for at least onehour. After evaporation of the petroleum spirit, the oxide powder waspressed into 1.2 mm thick pellets (φ=8 mm, pressing pressure >40 kN).The green bodies thus produced were first ignited at a temperature of500° C. in a corundum boat in a pure O₂ atmosphere for a period of 12hours and then heated as quickly as possible to 905° C. (firingtemperature). The temperature was here raised from 800° to 905° C.within 12 minutes. After reaching the final temperature, temperaturemeasurement and regulation were carried out by means of the second innerthermocouple. Cooling was carried out at a rate of 6K/min to atemperature of from 710° to 720° C. and finally after a further 10.5hours as quickly as possible to ambient temperature. It was establishedby thermoanalysis that a mass loss of about 2.5% by weight occurredduring the second heating phase in the temperature range above 500° C.

The transition temperature was measured on the shaped part produced inthis way:

a) AC susceptometer T_(c) =120K (intrinsic).

b) DC measurement T_(c) =111K (downset).

EXAMPLE 2

Corresponding to the formula Tl₀.8 Ca₂.2 Ba₁.9 Sr₀.1 Cu₃ O₈.8+x, 32.15parts by weight of Ba(OH)₂ 8 H₂ O, 8.74 parts by weight of Ca(OH)₂, 1.43parts by weight of Sr(OH)₂ 8 H₂ O and 12.8 parts by weight of CuO wereweighed out, mixed and first pretreated as in Example 1. 9.8 parts byweight of Tl₂ O₃ were then added and pellets were pressed as inExample 1. The reaction of the pellets was carried out in a similarmanner to that described above in Example 1. The mass loss determined bythermoanalysis (DTA/TG measurement) during the second heating phase,above 500° C., was 2.9% by weight.

Measurement of the transition temperature on the shaped parts fromExample 2 gave the following result:

a) AC susceptometer T_(c) =119K (intrinsic).

b) DC measurement T_(c) =114K (downset).

EXAMPLE 3

Corresponding to the formula Tl₀.69 Pb₀.025 Ca₂.2 Ba₁.78 Sr₀.1 Cu₃O₈.5+x, 16.28 parts by weight of Ca(OH)₂, 56.1 parts by weight ofBa(OH)₂ 8 H₂ O, 2.6 parts by weight of Sr(OH)₂ 8 H₂ O and 23.9 parts byweight of CuO were weighed out and treated as in Example 1. Afterignition for three hours at from 600° to 650° C., 0.67 parts by weightof lead were added as Pb(NO₃)₂ to the still thallium-free oxocupratemixture. Further processing is carried out as described in Example 1.18.26 parts by weight of Tl₂ O₃ were added to the oxide mixture heattreated at 950° C. and the resultant mixture was ground in a ball millas in Example 1. The oxide powder from the ball mill was pressed into anumber of larger pellets having a thickness of 3.5 mm and a diameter of20 mm (pressing pressure 100 kN). The first reaction step to give thesuperconductor was as in Example 1. In a second step, the pellets wereagain ground up, 3% by weight of PbO were added and the mixture wasground in a ball mill for 1 hour. For the second firing process, themobile furnace was preheated beforehand to 1000° C. and only then pushedover the part of the tube containing the corundum boat and the sample.From then on temperature control and regulation were carried out bymeans of the inner thermocouple. The final temperature (about 905° C.)was held for a further 10 minutes. Pure oxygen flowed through the samplechamber, with the flow rate being set to 5 l/h. The cooling phasecorresponded to that of Example 1.

Result

a) AC susceptometer curve after step 1→T_(c) =95K.

b) AC susceptometer curve after step 2→T_(c) =110K.

EXAMPLE 4

The oxide mixture of the nominal composition Tl₀.8 Ca₂.2 Ba₁.9 Sr₀.1 Cu₃O₈.5+x, produced as in Example 2, was pressed into larger pellets andfired as described in Example 3. For the second reaction step, thesample which had been again ground up in the ball mill was pressed intobars (approximate dimensions 5×5×40 mm, 16 kN). The second firingprocess corresponded to that of Example 3 with a further ignitionprocess (time: 12 hours) in the temperature range from 712° to 720° C.(O₂ stream: 5 l/h). This second firing process was repeated again. Thebar had then shrunk by about 10% to a length of 36.5 mm. The densitycalculated from the dimensions and the mass was 4.3 g/cm³. It was splitin the longitudinal direction by means of a metal saw and ground to thedimensions 2.5×2.5×36 mm using a grinding disc ®ECOMET 2 (from BuehlerLtd.). Silver contacts (layer thickness from 60 to 80 mm) were thenapplied in a ®SPUTTER COATER S150B (from EDWARDS) and fired at 500° C.for 20 minutes in an oxygen atmosphere.

Result

a) T_(c) =118K (AC curve and SQUID).

b) According to X-ray powder data, the sample contained primarilyTl-1223, BaCuO₂, a little Tl-1122 and Ca₂ CuO₃ and no Tl-2223.

c) Volume fraction of superconductor 43% by volume (from powdermeasurement--SQUID--estimation).

d) Zero-field critical current density: J_(c) =460 A/cm².

To measure the critical current density, the test specimen men wasmeasured using the customary four-point method. For this purpose, a barwas clamped in a heavy current generator designed for a current of 500A. The current was fed in via the outer ends of the bar and the voltagewas measured at two contacts 3 cm apart on the rod. The critical currentdensity is calculated from the current which causes a voltage drop ofexactly 1 μV/cm.

EXAMPLE 5

A sample having the nominal composition Tl₀.8 Ca₁.2 Ba₂ Cu₂ O₆.5+x wasweighed out from the mixture of oxocuprates precalcined as in Example 1and Tl₂ O₃. The pellets having a diameter of 8 mm were first precalcinedfor 8 hours at 500° C. in a stream of pure oxygen. The temperature wasthen increased at a rate of 6K/min to 780° C. and held for 5 hours. Thiswas followed by a heat treatment process at 710° C. for a time of 12hours.

Result

a) T_(c) =93K (AC curve and SQUID).

b) According to measurements using the SQUID magnetometer, the sampleadditionally contained a second superconducting phase having a T_(c) of60K (presumably Tl-1021).

We claim:
 1. A process for producing high-T_(c) superconductorscontaining thallium, calcium, barium and copper, and optionally leadand/or strontium, comprisingpreparing a thallium-free precursor in afirst reaction step by mechanically triturating a mixture of compoundsof the metals Ba, Ca and Cu, and optionally Pb and/or Sr, having a metalcontent in the numerical ratio of atoms desired in the particular case,heating the mixture to temperatures in the range from 700° to 950° C.and heat treating the mixture for a period of at least 3 hours, andthen, optionally cooling the mixture to ambient temperature, grindingthe mixture and subsequently heat treating the mixture at temperaturesof from 400° to 500° C. in a stream of pure oxygen, said first reactionstep being conducted using alkaline earth metal hydroxides, andtritrating the precursor with Tl₂ O₃ optionally, shaping the precursorinto a shaped part, and oxidatively firing the precursor triturated withTl₂ O₃ in a flowing gas atmosphere.
 2. The process as claimed in claim1, wherein the oxidative firing of the triturated Tl₂ O₃ /precursor iscarried out in a mobile tube furnace in a flowing, oxidizing gasatmosphere at a flow rate of>3 l/h and at a heating rate of at least 30K/min.
 3. The process as claimed in claim 2, wherein the gas used is airor pure oxygen.
 4. The process as claimed in claim 1, wherein prior tomechanical trituration in the first reaction step, a mixture of Ba(OH)₂×8 H₂ O, Ca(OH)₂ and copper oxide, and optionally lead oxide or leadnitrate and/or Sr(OH)₂ ×8 H₂ O, having a metal content in the numericalratio of atoms desired in each case, is heated at temperatures of atleast 130° C. for a period of at least 1 hour and wherein a seconddrying process is then carried out at at least 400° C. for a period offrom 1 to 3 hours.
 5. The process as claimed in claim 1, whereincompounds having the nominal composition Tl/PbCa₂ Ba₂ CuO₈.5+x areheated during oxidative firing of the triturated Tl₂ O₃ /precursor attemperatures of at least 900° C. with the firing temperature beingmaintained for a time of at most 30 minutes.
 6. The process as claimedin claim 1, wherein compounds having the nominal composition Tl/PbCaBa₂Cu₂ O₆.5+y are heated during oxidative firing of the triturated Tl₂ O₃/precursor at temperatures of from 760° to 790° C., with thistemperature being held for a time of at least 3 hours.
 7. The process asclaimed in claim 1, wherein the oxidative firing of the triturated Tl₂O₃ /precursor is followed by an additional further heat treatment atabove 700° C. for a time of at least 5 hours.
 8. The process as claimedin claim 1, wherein the oxidative firing of the triturated Tl₂ O₃/precursor including heat treatment and cooling is repeated a number oftimes.
 9. The process as claimed in claim 1, wherein subsequent tooxidative firing of the triturated Tl₂ O₃ /precursor, the material isground once more, mixed with from 1 to 3% by weight of Tl₂ O₃, andoptionally mixed with lead oxide or PbNO₃, pressed and fired again. 10.The method of claim 1, further comprising smoothing the surface of theformed high-Tc superconductor, and subsequently etching and sputteringsilver on said surface.
 11. A process as claimed in claim 1, whereinsaid superconductor prepared has minimal thallium loss without the useof quartz ampoules or gold foils.
 12. A process as claimed in claim 2,wherein said heating rate is at least 60 K/min.
 13. A process as claimedin claim 5, wherein said oxidative firing of the triturated Tl₂ O₃/precursor is conducted at a temperature of at least 905° C., with thefiring temperature being maintained for a time of at most 20 minutes.14. A process as claimed in claim 7, wherein said additional furtherheat treatment is at above 700° C. for a time of from 8 to 16 hours. 15.A process as claimed in claim 8, wherein said oxidative firing of thetriturated Tl₂ O₃ /precursor is repeated two to three times.
 16. Aprocess as claimed in claim 1, wherein prior to mechanical triturationin the first reaction step, a mixture of the alkaline earth metalhydroxides is heated at a temperature of at least 130° C. for a periodof at least 1 hour.
 17. A process for producing high-T_(c)superconductors containing thallium, calcium, barium and copper, andoptionally lead and/or strontium, comprisingpreparing a thallium-freeprecursor in a first reaction step employing compounds having thenominal composition Tl/PbCaBa₂ Cu₂ O₆.5+y that are fired at firingtemperatures of from 760° to 790° C., with this temperature being heldfor a time of at least 3 hours, titrating the precursor with Tl₂ O₃optionally, shaping the precursor into a shaped part, and oxidativelyfiring the precursor triturated with Tl₂ O₃ in a flowing gas atmosphere.